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#1 |
New Member
Join Date: Mar 2020
Posts: 4
Rep Power: 6 ![]() |
Hi everyone,
Im a beginner in CFD. For the last assignment in my university i have to create a reliable Ahmed body model, using SST model. This means that the y+ should be around 1 as much as possible. After few iterations on the first cell thickness, the distribution of y+ is pretty much ok, but there is two really specific regions where the y+ goes to 0, or very close to, everytime, and i didn't figured out why. If someone has a clue of what is happening i would really appreciate it ! Here is what i am talking about : https://www.casimages.com/i/200308014028682006.png.html Thanks in advance for all the people taking the time to answer me. |
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#2 |
New Member
Arnie
Join Date: Mar 2017
Posts: 27
Rep Power: 9 ![]() |
This is due to the low velocity (air flow) at areas of re-circulation....similar phenomenon as on the sloped rear end.
Further, low Y+ values can be seen at the flat front face of the Ahmed body due to the fact that the air flow has practically come to a stop. Regards |
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#3 |
Member
Join Date: Mar 2009
Posts: 36
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As mentioned, it appears to be localised recirculation or BL separation.
Its been years since I've looked at Ahmed so could be very wrong here, but I don't recall there being separated zones ahead of the sloped rear - the whole point of the model was to examine that aspect of turbulence models. (i) Are you running at a very subsonic air speed (ii) What is your surface roughness? (iii) Is the nose definitely fully blended to the main body (no sharp curvature changes)? |
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#4 |
New Member
Join Date: Mar 2020
Posts: 4
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Hi,
We are running the simulations with a Reynolds of 30 000 with water (imposed), so with the frontal area of the car as characteristic length the average velocity is around 0.3 m/s so yes, that is a very subsonic air speed. I honestly have no idea about the surface roughness, but we have imposed a no slip condition on all surfaces of the car (maybe is that a bad idea ?). Finally we made the geometry with a simple sketch extruded so yes the nose should be fully blended to the main body. Im wondering if these low y+ are really that dramatic, because i can't see how i can fix it. Thanks again for your answers. |
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#5 |
Member
Join Date: Mar 2009
Posts: 36
Rep Power: 17 ![]() |
No, no - no-slip is fine.
Looks like your fine as your running in water: https://www.researchgate.net/publica...Porous_Devices This is run in water at Re 30000, and there are circulation zones in around the areas you see the Y+ deviations. Now - obviously that is still just comparing numerical to numerical rather than experimentals - but it gives some confidence in what your doing. But I know Bordeaux have good water tunnels with experienced people running them, so its unlikely they would not have some good internal data to compare against. |
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#6 |
Senior Member
Join Date: Nov 2015
Posts: 246
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If you use k-w or k-w SST turbulence models then you don’t have min y+ bound. There are no sudden flow changes at y+ <1. At range 0<y+<5 flow is assumed to be laminar.
https://en.wikipedia.org/wiki/Law_of_the_wall https://www.youtube.com/watch?v=fJDYtEGMgzs ![]() |
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#7 |
New Member
Join Date: Mar 2020
Posts: 4
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Hi,
Indeed it seemed to be caused by water and the re circulation areas, which i can not do anything about it. Thanks to everyone who took the time to answer and help, you guys are awesome ! |
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Tags |
ahmed, body |
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